The present invention relates to an improvement in a solid-state image pickup device.
More solid-state image pickup devices are currently being used as image sensors than conventional image pickup tubes. A solid-state image pickup element generally comprises a matrix of integrated semiconductor photoelectric conversion cells. Therefore, the solid-state image pickup element is light in weight, compact in size, high in vibration resistance, consumes less power, and is high in reliability. The arrangement of the photoelectric conversion cells, that is, the arrangement of the picture elements of the light-receiving surface, is fixed, and the clock pulses for obtaining outputs from the respective cells are also specifically defined. For this reason, if the distortion introduced by the optical system is excluded, the figurative distortion of the resultant image is very small.
If such a solid-state image pickup element is used for a television camera, the reading frequency of the image data for one frame is fixed at 1/30 sec. The amount of light incident on the image pickup conversion cell is generally set to be an intermediate value between the saturation level of the photoelectric conversion cell and the dark current level. If the reading period of the image data is relatively short and is constant, the amount of light incident on the element may remain at such a value. However, if the reading frequency changes irregularly, as in the case of exposure control through adjustment of the exposure time, the amount of light on the element must be varied in accordance with such an irregular change. If the reading period of the image data is very long, the intensity of light incident on the image pickup element must be made extremely small. In such a case, the exposure or the integrated value of the amount of incident light rays as a function of time changes during reading of the image data. Accordingly, a photoelectric conversion output from the initial cell differs significantly from that from the last cell. Uniform photoelectric conversion sensitivity cannot therefore be obtained over the entire surface of the image pickup element.